Adiabatic Small Polaron Hopping Research Articles

Ferroelectricity in the glassy material of the composition Bi 2O 3–Pb 3O 4–CuO–K 2O

Glass sample of the composition 31.4 Bi 2O 3–2.33 Pb 3O 4–64.53 CuO–1.74 K 2O in mol% was prepared by the conventional quenching melt technique. The as-quenched single phase glass shows interesting ferroelectric properties which is not known in the field of glass science. If the as-quenched glass is heat treated above the glass transition temperature all signs of ferroelectricity disappears completely. X-ray diffraction and transmission electron microscopy as well as differential thermal analysis were used to recognize the glassy nature of the as-quenched sample. Ac dielectric measurements were performed as a function of temperature and frequency and showing ferroelectric to paraelectric phase transition at Curie's temperature of 540 K. Non-linear polarization as a function of temperature and applied electric field as well as pyroelectricity were also studied. The conduction mechanism was confirmed to obey the adiabatic small polaron hopping (SPH) and was mainly due to electronic transport between Cu ions. The dominant factor determining conductivity was the hopping carrier mobility in this glass. From the best fits, reasonable values of various SPH and VRH parameters are obtained.

Effect of the simultaneous substitution of two transition metals on the structural, magnetic and electrical properties of La 0.6Sr 0.4Mn 1−2 xCr xFe xO 3

The structural, magnetic and electrical properties of Cr and Fe simultaneously substituted in the perovskite La 0.6Sr 0.4Mn 1−2 x Cr x Fe x O 3 have been studied. The presence of Cr and Fe had no significant effect on the structural properties. Curie temperature and saturation magnetization decrease with increase in Cr and Fe contents. For x=0.20 and 0.25, a steep drop of zero field-cooled (ZFC) magnetization at low temperature signifies the formation of cluster- or spin-glass state. A weak hysteresis at low fields seems to be an indication of phase separation. All the resulting magnetization curves can be explained by a superposition of both ferromagnetic and antiferromagnetic components. All the samples are semiconducting throughout the temperature range studied. Resistivity can be described by the adiabatic small polaron hopping and the variable range hopping model. It was found that the transport mechanism is dominated by the VRH model with an increase of Mott localization energy, which explains the increase of resistivity.

Cu Doping Effect on Electrical Resistivity and Seebeck Coefficient of Perovskite-Type LaFeO3 Ceramics

Perovskite-type LaFe1–xCuxO3 (x = 0.10, 0.14, 0.18) solid solution is prepared with the conventional solid-state reaction technique. The electrical resistivity and the Seebeck coefficient are measured in the temperature range 473–1073K to elucidate the Cu doping effect on the thermoelectric properties of the LaFeO3. The electrical resisitivity of LaFe1–x Cux O3 shows semiconducting behavior. The temperature dependence of the electrical resistivity indicates that the adiabatic small-polaron hopping mechanism is dominant for their electric transportations. The activation energy decreases with the increasing Cu content as well as the increasing temperature. The Seebeck coefficient changes from a negative value to a positive value around 510 K, and increases with rising temperature up to 710K, then becomes saturated around 200μV/K. The Seebeck coefficient decreases with the substitution of Cu atoms in the temperature range of 573–1073 K, while the electrical resistivity decreases with the substitution of Cu atoms in the whole measured temperature. Overall the power factor increases with rising temperature, and the highest value of power factor is 54 μW/K2 m for x = 0.10 of Cu doping.

Low temperature resistivity minimum and its correlation with magnetoresistance in La 0.67Ba 0.33MnO 3 nanomanganites

A systematic investigation of structural, magnetic and electrical properties of nanocrystalline La 0.67Ba 0.33MnO 3 materials, prepared by citrate gel method has been undertaken. The temperature-dependant low-temperature resistivity in ferromagnetic metallic (∼50 K) phase shows upturn behavior and is suppressed with applied magnetic field. The experimental data (<75 K) can be best fitted in the frame work of Kondo-like spin-dependant scattering, electron–electron and electron–phonon interactions. It has been found that upturn behavior may be attributed to weak spin disorder scattering including both spin polarization and grain boundary tunneling effects, which are the characteristic features of extrinsic magnetoresistance behavior, generally found in nanocrystalline manganites. The variation of electrical resistivity with temperature in the high temperature ferromagnetic metallic part of electrical resistivity (75K< T< T P) has been fitted with grain/domain boundary, electron–electron and magnon scattering mechanisms, while the insulating region ( T> T P) of resistivity data has been explained based on adiabatic small polaron hopping mechanism.

Structural and magnetic ordering inLa0.5Ca0.5−xBaxMnO3 (0 < x ≤ 0.5) manganite

We report a structural, magnetic and transport study inLa0.5Ca0.5−xBaxMnO3 (0<x≤0.5) using neutron diffraction, magnetization and resistivity measurements.The samples crystallize in an orthorhombic structure withPnma space group and no structural transition is observed with Ba doping.Substituting Ca with Ba leads to an increase in average A-site ionic radii,⟨rA⟩ and cationsize disorder (σ2). Consequently, the nature of magnetic ordering undergoes changes as a function ofx. At lowtemperature for x≤0.1, the CE-type antiferromagnetic spin structure is stabilized, similar to the parent compound. Atx = 0.15, the magnetic phase is a mixture of CE-type and A-type spin structure. Forx≥0.2, the ferromagnetic phase is stabilized. The signature of orbital orderingin addition to spin and charge ordering is observed in all samples withx≤0.15.TC,TCO andTN remain nearlyconstant for x≤0.2. The transport studies reveal that for samplesx≤0.2, adiabatic hopping of small polarons governs the transport behaviour aboveTCO (∼160 K). A parallel combination of small polaron hopping and variablerange hopping mechanisms is found to describe resistivity data belowTCO. Atx = 0.2, an insulator to metal transition is observed as temperature is lowered, and atx = 0.5 the sample becomes metallic below 280 K. In the metallic regime, resistivity data followsthe Bloch–Grüneisen model together with contributions from high frequency optical phononsand magnetic scattering. The experimentally established phase diagram from the presentstudy is compared with theoretical studies in the intermediate electron–phonon interactionregime.

The Effect of Co Substitution on the Crystal Structure and Electrical Resistivity of (La0.85Ag0.15)MnO3 Compounds

The effect of Co substitution on the crystal structure and electrical transport properties of La0.85Ag0.15Mn1−yCoyO3 compounds (0≥y≤0.50) has been studied. Structural transition from rhombohedral to orthorhombic symmetry has been observed with Co doping. The lattice parameters are found to increase with doping up to y=0.20, then it decreases. It is explained on the basis of transition from high spin state to low spin state of Co ions with increase in doping beyond y=0.20. Ferromagnetic (FM) metallic behavior with colossal magneto-resistivity has been observed up to y=0.10. However, for y≥0.15 compounds, the temperature dependence of resistivity ρ(T) follows semiconducting behavior. The electrical resistivity in the metallic region could be explained based on electron–electron and electron–magnon scattering mechanisms. The data in the semiconducting region could be explained based on the variable range hopping model for y=0.2 and adiabatic small polaron hopping model for y≥0.3.

Adiabatic small polaron hopping conduction in La0.7Nd0.3Mn0.8Cr0.2O3

Abstract The dc conductivity and magnetization of insulating La 0.7 Nd 0.3 Mn 0.8 Cr 0.2 O 3 were measured in temperature ranges of 77–300 and 4.2–300 K, respectively. La 0.7 Nd 0.3 Mn 0.8 Cr 0.2 O 3 shows a ferromagnetic transition around 130 K but the sample is insulating throughout the temperature range studied. The temperature dependence of conductivity shows that electronic transport in La 0.7 Nd 0.3 Mn 0.8 Cr 0.2 O 3 is well described by the Emin–Holstein adiabatic small polaron hopping model. The polaron density extracted from conductivity measurements is about 3.0×10 20 cm −3 . This result agrees well with the nominal polaron density for La 0.7 Nd 0.3 Mn 0.8 Cr 0.2 O 3 , which is also roughly 3.0×10 20 cm −3 . We also derived important quantities for transport in La 0.7 Nd 0.3 Mn 0.8 Cr 0.2 O 3 .

High temperature thermoelectric characteristics of Ca0.9R0.1MnO3 (R=La,Pr,…,Yb)

Electron-doped perovskite manganite Ca0.9R0.1MnO3 (R=La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Yb) polycrystalline samples were prepared and their transport and thermoelectric properties were studied from room temperature to 1000 K. The transport behavior for all the samples is adiabatic small polaron hopping mechanism below 600 K but changes to metallic conductivity at higher temperature. Above 600 K, more 3d electrons of Mn3+ ions will occupy eg orbitals, resulting in the variation in thermoelectric power values. For all the samples, thermoelectric power is only determined by carrier concentration, but resistivity also rests with effective bandwidth. The size matching between Ca2+ and R3+ ions together with heavier R3+ doping can improve thermoelectric performance evidently. Combining these two factors, Ca0.9Dy0.1MnO3 and Ca0.9Yb0.1MnO3 reach ZT=0.2 at 1000 K, suggesting that they can be efficient high temperature n-type thermoelectric oxide materials.

Electrical transport and photoinduced properties in La0.7Sr0.3CrO3 thin film

Abstract Direct current (dc) electrical transport and photoinduced properties of the high-quality epitaxial thin film of the hole-doped chromium oxide La 0.7 Sr 0.3 CrO 3 on a (1 0 0)-oriented LaAlO 3 substrate have been studied. The sample exhibits semiconductive behavior and the electrical transport mechanism agrees with the adiabatic small polaron hopping model. The laser irradiation results in a decrease of the resistivity of the film in the whole temperature range measured, which is attributed to the delocalization of the small polarons.

Polaron liquid-gas crossover at the orthorhombic-rhombohedral transition of manganites

High-resolution synchrotron x-ray powder diffraction in ${\text{La}}_{0.7}{\text{Ca}}_{0.3}{\text{MnO}}_{3}$ shows in detail a first-order structural phase transition from orthorhombic (space-group $Pnma$) to rhombohedral (space-group $R\overline{3}c$) crystal structures near ${T}_{S}=710\text{ }\text{K}$. Magnetic susceptibility measurements show that the rhombohedral phase strictly obeys the Curie-Weiss law as opposed to the orthorhombic phase. A concomitant change in the electrical resistivity behavior, consistent with an alteration from nonadiabatic to adiabatic small polaron hopping regimes, was also observed at ${T}_{S}$. The simultaneous change in transport and magnetic properties are identified as a transition from a correlated polaron liquid for $T&lt;{T}_{S}$ to an uncorrelated polaron gas for $T&gt;{T}_{S}$, driven by the change in the crystal symmetry.

High-Temperature Electrical Transport and Thermoelectric Properties of La0.75Ba0.25MnO3 Ceramics

In this study, the thermoelectric power and resistivity of the perovskite manganite <TEX>$La_{0.75}Ba_{0.25}MnO_3$</TEX> were investigated in the temperature range 300K-1200K. The electrical resistivity and thermoelectric power indicate a transport mechanism dominated by adiabatic small-polaron hopping. The power factor increases from <TEX>$2{\times}10^{-6}W/mK^2$</TEX> to <TEX>$1{\times}10^{-5}W/mK^2$</TEX> as to the temperature increases from 400K to 1200K, which indicates that the compound is highly feasible as a thermoelectric material at high temperatures.

Electrochemical reduction of NO and O2 on La2−x Sr x CuO4-based electrodes

A series of La2 − x Sr x CuO4 (x = 0.0, 0.05, 0.15, 0.25 and 0.35) compounds was investigated for the use of direct electrochemical reduction of NO in an all-solid-state electrochemical cell. The materials were investigated using cyclic voltammetry in 1% NO in Ar and 10% O2 in Ar. The most selective electrode material was La2CuO4, which had an activity of NO reduction that was 6.8 times higher than that of O2 at 400 °C. With increasing temperature, activity increased while selectivity decreased. Additionally, conductivity measurements were carried out, and the materials show metallic conductivity behavior which follows an adiabatic small polaron hopping mechanism.

Crystal structure, magnetic and electrical transport properties of SrKFeWO 6 double perovskite

The crystal structure, magnetic and electrical transport properties of SrKFeWO 6 double perovskite have been investigated. The structure of this double perovskite compound is assigned to monoclinic system with space group P2 1/ n and structural analysis suggests that cationic Fe and W alternately occupy B′- and B′′-sites. Thermal magnetization demonstrates that the magnetic transition temperature is higher than 500 K. Isothermal magnetization shows that the net magnetic moment per unit formula in the direction of the magnetization is 0.72 μ B (300 K). The electrical transport process can be described by non-adiabatic small polaron hopping model at low temperature, while the electronic transport behavior at high temperature is described by adiabatic small polaron hopping model. Large magnetoresistance, − 0.89, can be observed under low magnetic field, 0.5 T, around room temperature.

Polaron variable range hopping in [formula omitted] thin films

The mechanisms of electrical conduction in TiO 2 - δ ( - 0.04 ⩽ δ ⩽ 0.2 ) thin films have been investigated. The samples were rf-diode reactive sputtered films deposited in O 2 + Ar gas atmospheres where changes in δ and film structure had been achieved by varying the O 2 flow rate and the substrate temperature. The electrical transport properties of these samples were investigated by measuring the conductivity as a function of temperature between 17 K and room temperature. At the temperature range between 200 and 290 K the best fit to the experimental data was obtained assuming a dependence characteristic of adiabatic variable range hopping. At lower temperature the activation energy for the conductivity tends to zero. The results suggest that the conduction mechanism is adiabatic small polaron hopping, which switches to conduction in a polaron band at low temperatures.

Transport and Magnetic Properties of Nanosized La 0.6Sm 0.1Sr 0.3MnO 3 +xCoO Composites

The composites La 0.6Sm 0.1Sr 0.3MnO 3 + xCoO ( x = 0.0, 0.1, 0.2, 0.3, 0.4 mol), named as A x samples, were synthesized by the sol-gel technique to derive homogeneous CoO-coated composites. CoO addition induces an increase of resistivity ( p) and a decrease of Curie temperature ( T c ), magnetization, and T p at which the p peak is located. It has been concluded that the resistivity below T p fits well with the equation ρ = ρ 0 + ρ 2 T 2 + ρ 4.5 T 4.5, indicating the importance of grain/domain boundary effects, the electron-electron scattering process, and the two magnon scattering process. On the other hand, the paramagnetic insulating region may be explained by using adiabatic small polaron hopping mechanism, thereby indicating that polaron hopping might be responsible for the conduction mechanism. Magnetoresistance results were explained by a two-level model of tunneling MR and percolation model.

Electrical behavior of some rare-earth-doped Nd 0.33Ln 0.34Sr 0.33MnO 3 manganites

A series of rare-earth-doped Nd–Sr–Mn–O manganites having the compositional formula Nd 0.33Ln 0.34Sr 0.33MnO 3, (where Ln=Gd, Eu, Sm, Nd and Ce) were prepared by citrate-based sol–gel route with ethylene glycol as a gelating reagent. After characterizing the samples by X-ray diffraction and scanning electron microscopy, a systematic study of magnetic susceptibility, electrical resistivity and magnetoresistance has been carried out over a temperature range 80–300 K. The variation of both electrical and magnetic transition temperatures with average ionic radii of A-site cation is explained. The electrical resistivity data were analyzed using different theoretical models and it has been concluded that at low temperatures (ferromagnetic metallic region) the resistivity may originate from grain/domain boundary, electron–electron scattering and two-magnon scattering effects, while in the paramagnetic insulating regime, the variation of resistivity with temperature may be explained by adiabatic small polaron and variable-range hopping mechanisms. The values of activation energies are found to decrease, while the density of states at the Fermi-level, N( E F) are increasing with increasing 〈 r A〉. A suitable explanation for the observed behavior is given.

Influence of A-site cation mismatch on structural, magnetic and electrical properties of lanthanum manganites

With a view to understand the influence of cation mismatch on magnetic as well as electrical behavior of colossal magnetoresistive materials, a systematic investigation of La 0.67D 0.33MnO 3 (D = Ca, Sr, Pb and Ba) manganite perovskites has been undertaken. The materials were prepared by sol–gel route sintering at 900 °C and after usual structural characterization by X-ray diffraction (XRD), their metal–insulator transition ( T P) as well as magnetic transition ( T C) temperatures were determined. The XRD data have been analyzed by Rietveld refinement technique. It has been concluded that apart from A-site cation, the size variance factor ( σ 2) also influences T P and T C values. The extrinsic nature of magnetoresistance in ferromagnetic metallic region is explained on the basis of inter-grain spin polarized tunneling effect. A systematic analysis of electrical resistivity data were carried out using theoretical models to understand the conduction mechanism. It has been concluded that the electrical resistivity data in the ferromagnetic (metallic) regime ( T < T P) may be explained by domain or grain boundary and single magnon scattering processes, while the adiabatic small polaron and variable range hopping models may be used to explain the resistivity data of high temperature paramagnetic insulating region ( T > T P).

Relation between EPR spectra and electrical conductivity of Pr 1−xPb xMnO 3 perovskites

The correlation between structure, Curie temperature ( T C), line width of EPR spectra and electrical conductivity of Pr 1− x Pb x MnO 3 ( x = 0.1 , 0.2, 0.3, 0.4, and 0.5) perovskites is discussed. It was shown that both adiabatic small polaron and variable range hopping models are good for description of conductivity in paramagnetic region but the first one is more suitable for interpretation of temperature dependence of EPR line width in temperature range 1.2 T C< T<1.3 T C.

Temperature dependence of the thermoelectric power of La 1−xK xMnO 3 compounds in light of a two phase model

The temperature-dependent resistivity and thermoelectric power (TEP) of monovalent (K) doped La 1− x K x MnO 3 polycrystalline pellets ( x = 0.05 , 0.10 and 0.15) between 50 and 310 K are reported. K substitution enhances the Curie temperature ( T C) from 260 to 309 K. Adiabatic small polaron hopping governs the resistivity as well as the TEP in the paramagnetic phase. In the ferromagnetic region, an expression containing the residual resistivity, electron–electron, and electron–magnon scattering term explains well the thermal variation of the electrical resistivity. The TEP at low temperatures in ferromagnetic region ( S lt) can be well represented with an expression containing the electron–magnon and spin-wave fluctuation scattering and confirms dominance of electron–magnon scattering. Both, resistivity and TEP data between 50 and 310 K are further examined in light of a two-phase model based on an effective medium approximation, in which the conductivity in the transition region is supposed to originate due to the mixing of the metallic region, embedded in a semiconductive polaronic matrix and is characterized by “mixing factor”. Our analysis confirms that the temperature dependence of the TEP of La 1− x K x MnO 3 compounds computed using the expression derived using effective medium approach, gives a fairly good description of the observed thermal variation of TEP in monovalent (K) doped lanthanum manganites between 50 and 310 K.

Polaron hopping properties of [formula omitted] ( [formula omitted

Using temperature- and field-dependent resistivity ρ ( T , H ) and Hall coefficient R H ( T , H ) measurements, we investigate the anomalous transport properties of single crystals of Eu 1 - x Ca x B 6 ( x = 0.2 – 0.995 ). The replacement of Eu with Ca invokes drastic changes in the ρ ( T ) and R H ( T , H ) although Eu and Ca are isoelectric. It is found that the Hall mobility, defined as | μ H | = R H ( H = 1 T ) / ρ , can be expressed by a non-adiabatic small polaron hopping model with four-neighbor hopping sites for Eu 1 - x Ca x B 6 ( x = 0.2 ) and with three-neighbor hopping sites for Eu 1 - x Ca x B 6 ( x = 0.9 ). The Hall mobility of Eu 1 - x Ca x B 6 ( x = 0.6 ) is expressed well by either the four- or three-sites hopping model. On the other hand, the DC conductivity of Eu 1 - x Ca x B 6 ( x ⩾ 0.95 ) is found to be described well by an adiabatic small polaron hopping model. Furthermore, the DC conductivity of Eu 1 - x Ca x B 6 ( x = 0.9 ) can also be described well by the adiabatic polaron hopping process. From the above observations, we surmise that the electronic transport properties of Eu 1 - x Ca x B 6 are dominated by the small polaron hopping mechanism. The hopping mechanism changes gradually from adiabatic for x ⩾ 0.9 to three-sites non-adiabatic for 0.9 ⩾ x ⩾ 0.6 and to four-sites non-adiabatic for x ⩽ 0.6 .